This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Non-covalent weak interactions play important roles in biological systems. Very recently, we reported the structure and reactivity of the Met16Phe mutant of the blue copper protein pseudoazurin (PAz) to investigate the effects of the pi-pi interaction observed in the unusual structure and reactivity of fern plastocyanin. The Met16 substituted mutants of PAz, in which several alkyl and aromatic groups are introduced close to the imidazole of the His81 ligand, have been constructed and characterized in order to probe the effects of the indirect weak interaction on the structure and function of the active site. Electronic absorption spectra of Met16Tyr, Met16Trp, and Met16Phe mutants indicated the smaller ratio of A460/A598 = ~0.3 as compared to that of wild type PAz, A460/A594 = 0.46. EPR spectra of the Met16Phe, Met16Tyr and Met16Trp mutants indicated the enhancement of the axial signal contribution. In this proposal, we will investigate the detailed structural and electronic structural information at the active site of Met16X pseudoazurin mutant proteins by XAS.